The frequency of oscillation of an oscillator is typically adjusted with a control voltage. Such an oscillation is referred to as a voltage controlled oscillator (VCO). In some implementations the control voltage varies the current source for one or all of a plurality of inverters connected in series with each other. In other implementations the control voltage varies the capacitance in an inductance/capacitance resonator.
In typical integrated circuit implementations the circuit element which has a capacitance which varies with a control voltage is a collector varactor. This may be a diode employed as a variable capacitor with the variation of junction capacitance dependent on the reverse bias voltage or a MOS varactor in which case the enhancement or depletion region under the gate of a surface field effect transistor varies as a function of the gate to drain/source voltage. The drain or source are typically connected together in such implementations. In both cases the capacitance varies in a non-linear fashion to changes in control voltage. The variation in the frequency of oscillation in turn also varies in a non-linear fashion to changes in the capacitance of the resonator (in fact it typically varies as 1√ (LC). The overall result is that the gain of the oscillator, which is defined herein as the ratio of (changes in frequency)/(changes in control voltage), varies with the applied control voltage. In many applications, such as frequency synthesizers for cellular telephones, phase-locked loops for telephony systems, such as SONET optical links, it is desirable to minimize this gain.
A phase locked loop typically comprises a phase detector which measures the phase offset between a reference clock, a charge pump which converts the detected phase difference to a source or sink current that varies in duration commensurate with the duration of the phase difference, a loop filter consisting of some combination of resistors and capacitors which accumulates the charge of the charge pump resulting in a varying voltage. This voltage is either directly fed to the oscillator or passed through a buffering circuit.
A low gain in the VCO is instrumental in optimizing the system characteristics, such as phase noise, output clock jitter, power supply noise immunity or PLL damping factor. If the gain of the VCO is reduced the capture range of the PLL is also reduced for a given range of control voltage. If after manufacture the oscillator has an offset between its desired frequency or frequency range of resonance and of the system (including the VCO and ancillary circuits) the integrated circuit becomes unusable.
The prior art has focused on VCO tuning methods that utilize on-chip solutions. For example, U.S. Pat. No. 6,137,372 to Welland and U.S. patent publication no. 2002/0033739 A1 to Bisanti et al address the problem of tuning the capture range of a voltage controlled oscillator (VCO) by adding or subtracting circuit elements, in both cases capacitance within the VCO itself. These methods allow the construction of a VCO that can be adjusted for manufacturing variation in the VCO, and they can compensate for variation in ambient conditions that the circuit is operating in, such as changes in temperature, but they have the drawback of abruptly changing the oscillation frequency of the oscillator when a capacitor is added or removed. This is inherently acknowledged in U.S. Pat. No. 6,211,745 column 14 line 51, where it states that “When the VCO 500 is used in such a system the method 800 preferably prevents calibration of the VCO during the time slots that voice/data is being received or transmitted”, and in U.S. Pat. No. 6,137,373 in column 9 line 49 where it states that “If desired, the discrete control 502 may continue to monitor the output frequency (fout) 102. If too great an error is detected, discrete control 502 may move the switch (SW) 512 back to select initial control node 510 and again modify the digital control word (Bc) 404 based on a desired procedure.” This abrupt change in frequency can be detrimental to overall system performance as a difference in frequency between the VCO and the input reference will produce a phase excursion in the output clocks until the PLL has re-synchronized.
In U.S. Pat. No. 5,912,595 the VCO is tuned in frequency with a control voltage that is switched to discrete levels by a D/A (digital-to-analog) converter. The switching is done to compensate for changes in temperature to minimize the variation in the output frequency of the VCO. Because this switching creates discrete voltage levels in the tuning voltage again each switch action will produce a step in the control voltage of the VCO, creating an abrupt change in VCO frequency with the attendant problems.